The subject matter disclosed herein relates to systems and methods for reducing modal coupling of combustion dynamics. In particular, the system and methods may be incorporated into a gas turbine or other turbomachine.
Gas turbine systems generally include a gas turbine engine having a compressor section, a combustor section, and a turbine section. The combustor section may include one or more combustors (e.g., combustion cans), each combustor having a primary combustion zone. A fuel and/or fuel-air (e.g., oxidant) mixture may be routed into the primary combustion zone through fuel nozzles, and the combustion zone may be configured to combust the mixture of the fuel and oxidant to generate hot combustion gases that drive one or more turbine stages in the turbine section.
The generation of the hot combustion gases can create combustion dynamics, which occur when the flame dynamics (also known as the oscillating component of the heat release) interact with, or excite, one or more acoustic modes of the combustor, to result in pressure oscillations in the combustor. For example, one mechanism of combustion instabilities may occur when the acoustic pressure pulsations cause a mass flow fluctuation at a fuel port which then results in a fuel-air ratio fluctuation in the flame zone. When the resulting fuel/air ratio fluctuation (e.g., heat release oscillation) and the acoustic pressure oscillations have a certain phase behavior (e.g., in-phase), a self-excited feedback loop results.
Combustion dynamics can occur at multiple discrete frequencies or across a range of frequencies, and can travel both upstream and downstream relative to the respective combustor. For example, the pressure waves may travel downstream into the turbine section, e.g., through one or more turbine stages, or upstream into the fuel system. Certain downstream components of the turbine section can potentially respond to the combustion dynamics, particularly if the combustion dynamics generated by the individual combustors exhibit an in-phase and coherent relationship with each other, and have frequencies at or near the natural or resonant frequencies of the components. In general, “coherence” refers to the strength of the linear relationship between two dynamic signals, and is strongly influenced by the degree of frequency overlap between them. In certain embodiments, “coherence” can be used as a measure of the modal coupling, or combustor-to-combustor acoustic interaction, exhibited by the combustion system.
Accordingly, a need exists to control the combustion dynamics, and/or modal coupling of the combustion dynamics and/or the combustor-to-combustor phase of the combustion dynamics, to reduce the possibility of any unwanted sympathetic vibratory response (e.g., resonant behavior) of components in the turbine system.